Such apparatuses are known as fuel-cell or electrolyzer-cell assemblies. In this case, the active cell is a fuel cell that converts chemical energy into electrical energy, or an electrolyzer cell that converts electrical energy into chemical energy. The arrangement of a plurality of plate-shaped cells next to each other is designated as a stack, in particular fuel-cell stack or electrolyzer stack.
In the case of a fuel-cell assembly, fuel and an oxidant are continuously fed to the cell. During the reaction of the two materials, a flow of electrons and thus electrical energy is generated. Conventional individual fuel cells generate a low voltage of about 1.2 V; however, in contrast to this, they generate a comparatively high current density of up to approximately 3 amp/cm2 of active reaction area, where the area information relates to the size of the active areas in a fuel cell. Since in modern membrane fuel cells comprising, for example a polymer electrolyte membrane (PEM) and pole plates resting thereon on both sides, these active areas can be greater than 100 cm2, such an individual fuel cell can supply a current of 300 Ampere and more at a direct current voltage of approximately 1.2 V. The resulting current is calculated as the product of the active area in cm2 and the maximum current density.
Since a direct-current voltage of 1.2 V is too low for many technical applications, very often, a plurality of cells are connected in series in conventional fuel cell assemblies so that the voltages of the cells are additive. Besides the electrical series connection it is also possible to implement a series connection of the supply structure so that fuel and oxidant are fed to a cell at the same time as fuel and oxidant are discharged from the upstream cell. In such an embodiment, particularly compact fuel cells can be produced. Alternatively, a parallel connection of the supply structure can also be employed.
Usually, conventional fuel cells have a flat, planar shape with a substantially rectangular base area so that the individual cells can be stacked parallel and next to each other or on top of each other. This results in a parallelepipedal overall structure whose dimensions depend on the number and the area of the cells. The individual cells are solidly connected to each other in the stack. In order to remove individual cells from such a firm bond in the event of a defect, the electrical connections and the feed lines and discharge lines of the fuel and the oxidant, and the discharge lines of reaction lines have to be detached. Furthermore, the entire fuel-cell stack has to be disassembled, i.e. the pressure plates and the bipolar plates have to be disassembled. In the course of this, the undamaged membranes of the good cells are often destroyed. Accordingly, removing an individual fuel cell from such a stack is only possible with considerable technical effort and is time-consuming.
In contrast, also known are modular fuel-cell stacks, where an individual fuel cell is removably inserted as a module into the fuel cell housing. Each fuel cell forms a closed unit.
For the function of fuel cell it is necessary that the pole plates exert pressure on the polymer electrolyte membrane or on a gas diffusion layer provided between the membrane and the pole plate. The pressure substantially effects the necessary electrical contacting between pole plates and gas diffusion layers so that electrons generated by the reaction in the fuel cell reach the cathode.
Different possibilities are known for applying pressure. For example, this can be done by crowning the pole plates, the plates being formed with a concave curvature. During the assembly of the fuel cell, the edges of the pole plates are connected to each other while being electrically insulated so that they are pulled together. This results in a crowning of the pole plates and the desired contact pressure. Alternatively, the contact pressure can be generated by clamps, in particular spring-steel clamps placed from the outside onto a fuel cell. Furthermore, pressing can also be carried out hydraulically. For this, the pole plates are very thin so that they are highly flexible. Pressure exerted from the outside onto the pole plates then causes a corresponding deformation of the pole plates, the pressure being transferred to the gas diffusion layers. For this, the housing of the fuel cell forms a pressure chamber in which a liquid or a gas, i.e. in general a pressurized medium, is contained. The fuel cells are positioned in the liquid so that during as superatmospheric pressure builds up in the pressure chamber, the liquid transfers the pressure to the pole plates. This constitutes a hydraulic pressing of the cells.
If a defective fuel cell of such a fuel cell is to be replaced, it is necessary to disassemble the cell. The overpressure in the pressure chamber has to be reduced and the medium has to be emptied out. Only then can the defective fuel cell be replaced. For this reason, changing a defect cell of the modular system is also labor-intensive and time-consuming so that repairing the fuel cell ultimately involves higher costs.